Influence of polyacrylic acid molecular weight and concentration on glass-ionomers

Objectives: Glass-ionomer (GI) restoratives are aesthetic and adhesive, which makes them ideal for the restoration of natural dentition, however, the mechanical properties are inadequate to withstand posterior masticatory forces. It has previously been shown that optimising the concentration and molecular weight of the acid element of GIs may result in improved mechanical properties The aim was to investigate the influence of increasing the molecular weight and concentration of polyacrylic acid in solution on the performance of a GI restorative.

Methods: A commercial GI restorative powder (Ionofil™ Molar; Voco, Cuxhaven, Germany) was prepared with a series of polyacrylic acid solutions consisting of varying molecular weights and concentrations. The 24 h mean compressive fracture strengths, Weibull moduli and mean elastic moduli were determined for groups of 30 cylindrical GI restorative specimens (6 mm height, 4 mm diameter) and the data was analysed using a one-way analysis of variance (ANOVA) and Tukey's test comparisons at P<0.05.

Results: The mean compressive fracture, mean elastic modulus and Weibull modulus for Ionofil™ Molar were 177±17MPa, 4.7±0.2GPa and 11.4±2.1. The one-way ANOVA and Tukey's test comparisons (P<0.05) highlighted a significant increase in the mean compressive fracture strength, mean elastic modulus and Weibull modulus with increasing polyacrylic acid molecular weight and concentration up to a point.

Conclusion: The performance in terms of mean compressive fracture strength, Weibull modulus and mean elastic modulus was improved by manipulating the molecular weight of polyacrylic acid. However, increasing the concentration and molecular weight of the acid element results in an increase in the viscosity of the liquid and therefore impacts upon the working characteristics on mixing. Further work is required to optimise the molecular weight and concentration of polyacrylic acid in solution whilst not compromising the working characteristics.